DISTINCT CYTOPLASMIC REGIONS OF THE GROWTH-HORMONE RECEPTOR ARE REQUIRED FOR ACTIVATION OF JAK2, MITOGEN-ACTIVATED PROTEIN-KINASE, AND TRANSCRIPTION

The GH receptor (GHR) is a member of the cytokine/hematopoietic growth factor family, and protein tyrosine phosphorylation has been implicat ed in the signaling cascade of these receptors. It was recently shown that the tyrosine kinase JAK2 is associated with the GHR. GH induces t he activation of JAK2, which phosphorylates itself and the receptor. M itogen-activated protein (MAP) kinase activation and transcriptional s timulation of specific genes, such as Spi 2.1, have also been reported to be induced by GH. To identify functionally important regions in th e cytoplasmic domain of the GHR, we compared the actions of the wild-t ype receptor, two truncated mutants, and one internal deletion mutant (similar to the intermediate Nb2 form of the PRL receptor) in transfec tants of the Chinese hamster ovary cell line. A region of 46 amino aci ds adjacent to the membrane was found to be sufficient for activation of both JAK2 and MAP kinases. This region contains a proline-rich sequ ence (box 1) conserved in the cytokine receptor family that is importa nt for signal transduction. For transcriptional activity, the C-termin al region of the GHR is required, and we found that the last 80 termin al residues contain sequences allowing activation of the Spi 2.1 promo ter. Tyrosine phosphorylation of the receptor also requires the C-term inal portion of the GHR cytoplasmic domain, and we found that GHR tyro sine phosphorylation appears to be linked to activation of the Spi 2.1 transcription pathway. Thus, the GHR could be composed of at least 2 functional regions: the 46 proximal amino acids required for activatio n of JAK2 and sufficient to stimulate the MAP kinase pathway, and an a dditional carboxy-terminal region necessary for transcriptional activa tion.